Greenhouses have long been used to grow different varieties of plants, including ornamental plants and fruit and vegetable producing plants. Greenhouses often include a structure with a clear roof and walls. Solar radiation warms the greenhouse's interior. The closed environment of a greenhouse presents certain challenges compared with outdoor production. Pests and diseases need to be controlled and irrigation is required to provide water. Greenhouses must also be climate controlled to compensate for extreme highs and lows of heat and humidity, and to generally control the environmental conditions such as the level of carbon dioxide (CO2).
Most greenhouses encourage airflow from outside to recirculate the air in the greenhouse. This is often done using HVAC systems that cycle air from inside and outside the greenhouse.
Recently, some greenhouses have begun using a semi-closed greenhouse design. The semi-closed greenhouses are mechanically cooled in the summer with a heat pump. The semi-closed greenhouse reduces ventilation, which in a healthy semi-closed system introduces fewer pathogens to the growing environment and requires less pesticide or chemical treatment, increases CO2 levels in the greenhouse that encourages growing, and from an energetic viewpoint, the semi-closed system is beneficial for (unilluminated) fruit-vegetables, tropical pot plants and cultivation where both heat and cold are needed.
As described in U.S. Pat. No. 8,706,617 to Houweling, one type of semi-closed greenhouse operates in different modes to control the temperature and environmental conditions within the greenhouse. In one mode, ambient air is drawn into the greenhouse, and in other modes, air from within the greenhouse is re-circulated. In still other modes, the system draws ambient air in combination with recirculation of air, and when ambient air is drawn in, it can also be cooled.
After drawing in the air, it is circulated through the greenhouse through vents (tubes). Fans or other mechanisms for drawing air may be arranged on the tubes to supply a flow of air volume to the tubes to cool the greenhouse during the expected elevated outside (external) temperatures and to heat the greenhouse during expected low temperatures.
But even the semi-closed system draws air from outside the greenhouse into the greenhouse, which air will contain pathogens. Further, most semi-closed systems employ a simple heat pump that allows for only coarse temperature and humidity adjustment. And finally, as shown in the Houweling patent, the semi-closed greenhouses require a climate control room or area to pre-treat the air before circulation, which area takes up potential growing space and requires separate construction.